Can anyone help, I want to return an ordered list based on forloop in Django using a field in the model that contains both integer and string in the format MM/1234. The loop should return the values with the least interger(1234) in ascending order in the html template.
Ideally you want to change the model to have two fields, one integer and one string, so you can code a queryset with ordering based on the integer one. You can then define a property of the model to return the self.MM+"/"+str( self.nn) composite value if you often need to use that. But if it's somebody else's database schema, this may not be an option.
In which case you'll have to convert your queryset into a list (which reads all the data rows at once) and then sort the list in Python rather than in the database. You can run out of memory or bring your server to its knees if the list contains millions of objects. count=qs.count() is a DB operation that won't.
qs = Foo.objects.filter( your_selection_criteria)
# you might want to count this before the next step, and chicken out if too many
# also this simple key function will crash if there's ever no "/" in that_field
all_obj = sorted( list( qs),
key = lambda obj: obj.that_field.split('/')[1] )
Related
I get a queryset object every time i want some data from models.
So when i say,
"items = Items.object.get(value=value)"
I get --
"<QuerySet [<Item-name>]>"
I have to iterate through the queryset object to get the data and I do that with
"items[0]"
Is there any way I can avoid this?
Edit: I meant "items = Items.object.filter(value=value)"
first of all items = Items.objects.get(value=value) does not return a queryset,
rather it returns an object of <Items: Items object (1)>
To get the first(or just one result) or last date from the object, do this Items.objects.first() or Items.objects.last()
To get the desired data without using its index position, then you can filter it like this Items.objects.filter(value=value)
You are mistaken. items = Items.object.get(value=value) will not give you a queryset, but an object. items = Items.object.filter(value=value)
would give you a queryset.
Filter method will always give you a queryset, because; in order to minimize the need of database hits, django considers you might add additional filters through your code. So if you not execute that queryset, e.g. by using list(your_queryset) django never hits the database.
# when you are using 'get' in your query, you don't need to iterate, directly get an access to the field values
try:
items = Items.object.get(value=value)
except Items.DoesNotExist:
items = None
if items:
print(items.value)
I have the following line:
test = MyModel.objects.filter(user=request.user)
The problem with this line is that it will retrieve the whole row. What if i want to retrieve the data from a certain column? For example, instead of the whole row, i'm trying to retrieve the column email
I tried this, but it's not working: email = test.email
You can use .values_list('email', flat=True) [Django-doc], like:
test = MyModel.objects.filter(user=request.user).values_list('email', flat=True)
Then test is a QuerySet that wraps strings. But usually this is not good software design. Usually one retrieves and stores User objects, not column values. A model can add a lot of extra logic that prevents that certain values are stored, and it sometimes contains extra logic to validate, clean, etc.
If a User has a single MyModel, you can just use .get(..) instead, like:
the_email = MyModel.objects.get(user=user).email
or with .values_list:
the_email = MyModel.objects.values_list('email', flat=True).get(user=user)
I looked at this thread, with an example of sorting dictionaries.
I have a dictionary of programme objects where the key is a programme object and the value is a lookup of the number of related Project objects.
def DepartmentDetail(request, pk):
department = Department.objects.get(pk=pk)
programmes = Programme.objects.all().filter(department=department).exclude(active=False).order_by('long_name')
combi = {}
for p in programmes:
prj = Project.objects.all().filter(programme=p)
combi[p] = str(len(prj))
return render(request, 'sysadmin/department.html',{'department': department, 'programmes': programmes, 'combi': sorted(combi.items())})
In the model, Programme returns a string 'long_name', so I believe that I am trying to sort a string key and a string value.
In the template I get to the keys and values as so,
{% for programme, n in combi %}
This gives me the error..
unorderable types: Programme() < Programme()
I don't really understand the error, in the python 3 documentation it states that the sorted() method accepts any iterable - So why does this happen?
I'm looking at collections.OrderedDict to solve the problem, but I want to know why this doesn't work.
Thanx.
Databases with index on columns are really good at sorting. There's almost never a need to sort on the client side. You can almost always do it in the server. The funny part it you apparently know how to do it too.
....exclude(active=False).order_by('long_name') # <--- this
Guess what, your data is already sorted there isn't a need to sort it again inside python!!
But there is a much bigger issue in your code. You are fetching a set of Project items and then looping through that set to retrieve them all over again one by one. So if you have 200 Project items you are doing 200 queries when one query does the job just as well. Just add select_related or prefetch_related depending on which direction you have the relationship.
Your code ideally should be something like this
department = Department.objects.get(pk=pk)
programmes = Programme.objects.all().filter(department=department).exclude(active=False).order_by('long_name')
return render(request, 'sysadmin/department.html',{'department': department, 'programmes': programmes,})
As far as I can see combi just contains duplicated data. The same thing can be accessed from programmes eg. programme.project_set.all()
(again this depends on which direction you have the relationship, your models are not shown)
Recommended reading: https://docs.djangoproject.com/en/1.10/ref/models/fields/#django.db.models.ForeignKey
The issue is that sorted expects a way to be able to order the items and by default there isn't any way to know how to order your objects. You can provide a key
sorted(combi.items(), key=lambda i: i.long_name)
I am trying to display a map of my data based on a search. The easiest way to handle the map display would be to serialized the queryset generated by the search, and indeed this works just fine using . However, I'd really like to allow for multiple searches, with the displayed points being shown in a user chosen color. The user chosen color, obviously cannot come from the database, since it is not a property of these objects, so none of the aggregators make sense here.
I have tried simply making a utility class, since what I really need is a somewhat complex join between two model classes that then gets serialized into geojson. However, once I created that utility class, it became evident that I lost a lot of the benefits of having a queryset, especially the ability to easily serialize the data with django-geojson (or natively once I can get 1.8 to run smoothly).
Basically, I want to be able to do something like:
querySet = datumClass.objects.filter(...user submitted search parameters...).annotate(color='blue')
Is this possible at all? It seems like this would be more elegant and would work better than my current solution of a non-model utility class which has some serious serialization issues when I try to use python-geojson to serialize.
The problem is that extra comes with all sorts of warning about usefulness or deprecation... But this works:
.extra(select={'color': "'blue'"})
Notice the double quotes wrapping the string value.
This translates to:
SELECT ('blue') AS "color"
Not quite sure what you are trying to achieve, but you can add extra attributes to your objects iterating over the queryset in the view. These can be accessed from the template.
for object in queryset :
if object.contition = 'a'
object.color = 'blue'
else:
object.color = 'green'
if you have a dictionary that maps fields to values, you can do things like
filter_dictionary = {
'date__lte' : '2014-03-01'
}
qs = DatumClass.objects.filter(**filter_dictionary)
And qs would have all dates less than that date (if it has a date field). So, as a user, I could submit any key, value pairs that you could place in your dictionary.
I am returning a JSON serialized queryset using the following queryset:
genome_parents = Genome.objects.filter(genes=cus_id)
where cus_id is the FK pointing to a companies table so I am retrieving all Genome objects related to the current working company. I return this data after a form has been posted via:
genome_parents = serializers.serialize('json', genome_parents, use_natural_keys=True)
However, I need the natural key for one of my foreign keys, but the id for another (both on the same model). So one field is displayed nicely, but the other isn't. So this does what I need except for one little thing, I need the plain id number so that I can pre-populate my FK form field.
One thought I had was to just append something like
genome_parents.append({'id':gene.id})
but that obviously doesn't work. Is there anyway I can augment the JSON so that I can include one more little piece of data (or change how I format the JSON)?
Greg
Just switch the order of the operations. And put the entire gene object into the list so it is properly serialized.
genome_parents = list( Genome.objects.filter(genes=cus_id) )
genome_parents.append(gene)
json_genome_parents = serializers.serialize('json', genome_parents, use_natural_keys=True)